In the minimally-invasive treatment of cardiac arrhythmias, the radiofrequency (RF) ablation catheter is the most commonly-used therapy tool, and is referred to as the gold standard in trials of new ablation catheter designs. One major challenge of RF ablation is to actively control the ablation settings during treatment. Currently, the therapist relies on his or her own expertise to determine the optimal parameters for ablation, such as power, temperature, and duration. Note that these settings vary largely, due to sizable intra-patient differences of thickness of the local heart wall, perfusion, blood pressure and velocity, heart rhythm, etc. Although a highly-skilled therapist is able to achieve successes with this approach, it is not always the case, and there are serious consequences for the patient when an error is made.
The two major therapy-related problems result from either the under- or over-heating of the site. In the case of under-heating, the tissue is not sufficiently coagulated or the ablation lesion is not deep enough to form the arrhythmia-blocking lesion desired by the therapist. This can lead to persistent or recurring symptoms in the patient, and the requirement for subsequent treatment(s), longer periods of hospitalization, and greater risks of stroke and embolism. Redo ablation procedures are more difficult to perform, since the already treated areas are very hard to discriminate from insufficiently treated ones. The other extreme, over-heating, either causes rupturing of the tissue at the treatment site, releasing potentially life-threatening particles into the blood stream, or causes damage to neighboring organs and tissues. In the case where other organs are affected, fistulas can develop and these are often life-threatening (e.g., a fistula in the esophagus has roughly a 75% mortality rate).
There is prior art suggesting that photoacoustic measurements are generally useful for bum depth assessment. See Talbert, R. J et al. “Photoacoustic discrimination of viable and thermally coagulated blood using a two-wavelength method for burn injury monitoring,” Physics in Medicine and Biology, vol. 52, no. 7, pp. 1815, 2007 (a multiple wavelength photoacoustic imaging method to discriminate coagulated and non-coagulated blood in a dermal bum phantom using statistical methods. The Talbert study finds a border between viable and necrotic skin tissue through photoacoustic imaging at two optical wavelengths. The necrotic tissue contains thermally coagulated blood which is visibly brown. The underlying inflamed tissue is characterized by the presence of viable, i.e., non-coagulated, blood which is red. Using planar blood layers, Talbert found the ratio of photoacoustic absorption at wavelengths of 543 nanometers (nm) and 633 nm, respectively, in non-coagulated blood to be 13.5:1; whereas, the ratio was 1.6:1 in coagulated blood. By statistical techniques, the border between the viable and necrotic condition of skin is located. There is also prior art suggesting that functional photoacoustic imaging is useful for hemoglobin oxygen saturation of single vessels. See Zhang, H. F. et al. “Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging”, Nature Biotechnology, Volume 24, Number 7, July 2006.